Physical vapor deposition is a well-known deposition process in which elements or molecules to be deposited on a substrate within a deposition chamber are supplied via evaporation or sublimation processes. While physically different processes, the words “evaporation” and “sublimation” are used interchangeably herein and depend only on the material to be deposited. The deposition chamber is enclosed and typically under vacuum. In other words, at least some measurable amount of gas is removed from within the deposition chamber. The deposition chamber is typically formed from steel, aluminum, other metal or glass and defines a physical boundary between an outside region, typically air, and the internal region wherein the gas is partially removed.
When gas is at a temperature above absolute zero Kelvin, each molecule or atom of gas has a specific kinetic energy defined by ½ mv2, where m is the mass of the gas particle (an atom or molecule) and v is the particle's velocity. When these energetic particles collide with the chamber walls, they exert a force on the chamber walls. This force exerted on the chamber walls is manifest as a pressure and is often described in units such as pounds per square inch (psi), pascals or torr among other units commonly known by those skilled in the art. The amount of force exerted depends on the area of the chamber, the number of collisions that occur, and, thus, the density and kinetic energy of the gas.
When one considers a system wherein the pressure outside of the chamber is different than that inside the vacuum chamber, a net force acts on the chamber walls. If the pressure outside the chamber is less than that inside the chamber, a force is exerted that tries to expand or even rupture the chamber. In this case, the chamber is referred to as a pressure chamber. If the pressure inside the chamber is less than the pressure outside of the chamber, a net force is exerted that tends to try and compress or even crush the chamber. In this case, the chamber is often referred to as a vacuum chamber and must be robustly built so as to withstand the forces exerted on it. From this point forward, for shorthand, the “chamber” will be referred to as a “vacuum chamber” or “chamber.”
In many applications, such as semiconductor, optical coatings, tool coatings, and various biomedical applications, the processing of products include, but are not limited to, thin film deposition, etching and annealing. It is, thus, desirable to not only evacuate a chamber, but to also introduce to the vacuum chamber a controlled flow of a known gas. This gas may supply a product used in the deposition, etching, annealing or, in general, used for processing the product produced in a vacuum environment. The vacuum may provide a means of processing not possible without the vacuum, such as plasma processing, or might merely provide a pressure differential so as to allow a gas to flow and thus be delivered to the product under process. Alternatively, the vacuum may provide a means of reducing undesirable background impurity concentrations so as to prevent contamination of the product or prevent undesirable chemical or thermal reactions. Those skilled in the art are aware of a multiplicity of reasons for using a vacuum to process a product, with these mentioned here are only given as examples and not intended to be exhaustive.
Since it is not typically possible to remove all of the gas particles in a chamber, some measurable amount of residual gas exists defining a vacuum chamber pressure as measured typically relative to atmospheric pressure at sea level (1 atmosphere equals 760 torr). When gas is removed such that the pressure is ˜<760 torr to ˜1×10−3 torr, the vacuum is said to be “medium vacuum.” When the pressure is further reduced to a range of ˜1×10−3 torr to ˜1×10−8 torr, the vacuum is said to be “high vacuum” or “HV.” When the pressure is further lowered to below ˜1×10−8 torr, the vacuum is said to be “ultra-high vacuum” or “UHV.”
The geometric size of the chamber, regardless of whether that chamber is a vacuum chamber, tube, orifice or any other enclosed volume, defines the certain key features of how the gas flows through a system. Thus, all enclosed volumes are simply referred to as a “chamber.” When gases are at sufficiently low pressure, the particles do not frequently collide with one another. The average distance for which the particles travel before they collide with each other is commonly known by those skilled in the art as the “mean free path” (between collisions). When the chamber walls are separated by distances less than the mean free path (λ), the collisions with the chamber walls dominate over inter-particle collisions in determining the resistance to gas flow via momentum transfer to and from the gas and chamber walls. This mode of gas flow is referred to as “molecular flow.”
When the mean free path of the gas is less than the dimensions of the chamber walls, the inter-particle collisions dominate the resistance to gas flow via momentum transfers from particle to particle. This mode of gas flow is known as “viscous flow” and behaves much like a fluid wherein the particles act to slow down and scatter other particles with lesser impedance effect from the chamber walls.
A gas can convert from one mode of flow to another as it travels through the vacuum system. For example, the gas can be delivered in a small tube (one form of a vacuum chamber) wherein its pressure and the tube dimensions define it to be in the viscous flow mode. The gas can subsequently be injected into a larger chamber, wherein its pressure and chamber dimensions define the gas to be in the molecular flow mode. A dimensionless quantity defined by the ratio of the mean free path to chamber major dimension known to those skilled in the art as the “Knudson number” (“K”), defines which mode the gas is in. If the Knudson number is greater than approximately 1, the gas tends toward molecular flow behavior, whereas if the Knudson number is less than approximately 1, the gas tends toward viscous flow. Those skilled in the art recognize that no clear boundary exists to define viscous vs. molecular flow and, thus, a region defined as “mixed flow” is often used to define the transition in flow characteristics when the Knudson number is approximately 1.
Often, the processing speed of a vacuum tool is determined in part by the “gas throughput” (“Q”), which is related to the molecular flux J. Throughput is a measure of the total mass flow through a system. Thus, higher mass flow equates to more gaseous species entering the chamber. In a deposition system, higher Q or J is desirable so as to increase the deposition rates and thus process throughput. Throughput Q is given in common units of torr-liters per second, standard (atmospheric pressure) cubic centimeters per second, standard liters per second or other units known to those skilled in the art. Molecular flux J is given in units of either number of atoms/(cm2-second) or grams/(cm2-second). In some cases, this throughput is defined at a given pressure at which the chamber is intended to operate, while in other cases, it is defined relative to atmospheric pressure (standard pressure).
The chamber pressure and the gas throughput are related by the chamber conductance C. The chamber, tube or orifice conductance is a measure of the inverse resistance to the flow of a gas and most often is measured in units of liters per second (L/S). Consequently, when in the molecular flow regime, the conductance is defined solely by the dimensions of the chamber, which act to restrict the flow with inter-particle collisions, having little effect on restricting the gas flow. Likewise, the conductance of the chamber in the viscous flow regime is dependent on the pressure of the gas, given that the gas pressure determines the number of collisions and, thus, the impedance to gas flow. In general, the relationship between Q, C and pressure P, isQ=CP,  Equation 1)where C is a constant dependent on chamber dimensions for molecular flow and C is a variable dependent on both chamber dimensions and on pressure for viscous flow. A more accurate equation for viscous flow considers the pressure dependence of the conductance resulting inQ=F(P2up−P2down),  Equation 2)where Pup is the pressure in the upstream flow (source of the gas flow) and Pdown is the pressure in the downstream of the gas flow (in the vacuum chamber or in the pump supplying vacuum to the chamber). The difference between the upstream and downstream pressures is often great enough that the downstream pressure can be ignored. Thus, Equation 2) can be reduced to Equation 1) by making C=F/(2×Paverage)˜F/(2×Pup).
A vacuum requires some sort of pump to remove the gas from the vacuum chamber to create the vacuum. The flow of gases into the chamber would result in a pressure increase unless a vacuum pump continuously removed the gas. Thus, every vacuum system contains at least one, often more than one, vacuum pump. The performance of a vacuum pump is described by a quantity known as the pumping speed S, and having units identical to that of the chamber conductance. Thus, the pumping speed of a pump is dependent on pressure in the viscous flow regime and independent of pressure in the molecular flow regime.
Since the pumping speed and the chamber conductance have the same unit, one can describe the combined effect of each by replacing the conductance C in Equations 1 and 2 by an effective conductance Ceff,1/Ceff=1/C+1/S.  Equation 3)
Likewise when several (n) chambers, large and small, as well as a pump are connected in a series gas flow arrangement, the effective conductance is found by1/Ceff=1/C1+1/C2+ . . . 1/Cn+1/S.  Equation 4)
When a pump having a high pumping speed is connected to a series of chambers of various dimensions, the lowest conductance chamber, i.e., the one with the smallest characteristic dimensions, will determine the overall conductance and, thus, dictate the throughput and pressure of the system. Thus, if a high-speed pump is connected through a small opening (limited conductance) to a vacuum chamber, the opening limits the overall conductance and, thus, the throughput and achievable pressure. This feature will prove important in understanding embodiments of the present disclosure, and will lead to ability to ignore the pumping speed S in Equations 3 and 4, resulting in the system flux throughput being determined by the pressure of the sources and the combined conductances of the delivery system (i.e., the effusion cell as described herein).
Under molecular flow conditions, the conductance is independent of pressure since the gas molecules are less likely to interact with one another. A circular orifice (an aperture with an opening that has a thickness substantially smaller than the diameter of the opening) has a conductance of:C=11.6πD2/4; orC=11.6(Cross-sectional Areaorifice).  Equation 5)
The conductance of a tube is:Ctube=11.6(D3/L); orCtube=11.6(Cross Sectional Areatube)(4D/πL),  Equation 6)where D is the orifice/tube inner diameter in cm, L is the tube length in cm, and P is pressure in torr. Note, for this molecular flow case, conductance C is independent of pressure.
For viscous flow for a tube, conductance C is given by:C=180(D4/L)Paverage; orC=180(Cross sectional Areatube)(4D2/πL)Paverage.  Equation 7)
A more accurate means of characterizing gas flow in the viscous flow regime than simply Q=CeffP isQ=F(P2upstream−P2downstream)  Equation 8)where Pupstream is the pressure upstream of the orifice and Pdownstream is the pressure downstream of the orifice, F is related to conductance, C by the relationship:F=C/(2Paverage), wherePaverage=0.5(Pupstream+Pdownstream).  Equation 9)
The relationship between C and F for the viscous flow tube can be proven as follows:
                    ⁢          Q      =              C        ⁡                  (                                    P              upstream                        -                          P              downstream                                )                                        ⁢                            P          average                ⁢        Q            =                        C          ⁡                      (                                          P                upstream                            -                              P                downstream                                      )                          ⁢                  P          average                                        ⁢                            P          average                ⁢        Q            =                        C          ⁡                      (                                          P                upstream                            -                              P                downstream                                      )                          ⁢                              (                                          P                upstream                            +                              P                downstream                                      )                    2                                        P                  average          ⁢                                                    ⁢      Q        =                  C        ⁡                  (                                    P              upstream              2                        -                                          P                upstream                            ⁢                              P                downstream                                      +                                          P                upstream                            ⁢                              P                downstream                                      -                          P              downstream              2                                )                    ⁢              1        2                                ⁢          Q      =                        (                      C                          2              ⁢                              P                average                                              )                ⁢                  (                                    P              upstream              2                        -                          P              downstream              2                                )                                        ⁢          Q      =                                    F            ⁡                          (                                                P                  upstream                  2                                -                                  P                  downstream                  2                                            )                                ⁢                                          ⁢                                          ∴                                          ⁢          F                =                  (                      C                          2              ⁢                              P                average                                              )                    
In Equations 5 through 7, all conductances are measured in L/Sec when dimensions are expressed in cm, pressure in Torr, and where the pre-factors to each of the above conductance equations account for the unit transformations. Non-circular apertures and tubes have similar expressions well known in the art but are less common.
The area dimensions of the openings of the tube and the orifice affect the conductance. Thus, for high mass flow as described by Equation 1), it is desirable to have large diameter tubes and orifices, maximizing the conductance.
Since embodiments of the present disclosure utilize several robust mechanical connections designed to support substantially larger mechanical loads than prior crucible and effusion cell designs, and since these connections might be required (in some cases) to be liquid tight so as to not allow liquefied evaporate material to escape, a means of joining thick and heavy parts in vacuum without creating a “virtual leak” may be required. A virtual leak is a pocket of trapped gas (not liquid) in a volume connected to the vacuum via a low conductance pathway. Such a pathway is often a small or highly constricted opening or a long narrow pathway such as a thread. Methods well known in the art to avoid virtual leaks include center boring bolts and screws to be used in blind tapped holes so as to provide an alternative gas pathway or slotting the female threads of a joint, allowing a more direct (straighter and shorter) path for gases to escape. Unfortunately, while slotting a female thread is necessary for allowing the trapped gas to escape, it also allows an opportunity for the liquid evaporate material to “wick” through the slot and escape the joint.
As is well known in the art, an MBE (molecular beam epitaxy) system consists of one or more heated crucibles filled with evaporate (or sublimation) materials configured in a vacuum chamber so as to direct the evaporate material toward a substrate to be condensed as a solid film. Mechanical shutters are typically placed outside of the crucibles so as to interrupt or allow the flux of evaporate material to deposit on the substrate. Crucibles are typically designed to be thin-walled materials (typically ˜0.035 inch thick) often made of Pyrolytic Boron Nitride (PBN) or other suitable high purity, heat transparent material or, in some cases, heat opaque refractory metals, oxides such as alumina, beryllium oxide, or graphite. In general, the crucibles can be removed and replaced, making them interchangeable and, thus, not material specific. Since it is desirable to adjust the evaporate flux to achieve various rates of deposition or to mix evaporate materials in specific flux ratios to produce composite thin films, in almost all cases, the MBE crucibles are thin so as to minimize the thermal mass and, thus, to maximize the response time of changing temperatures and, thus, changing the desired evaporate flux.
In any thin film deposition system, but particularly in MBE, the upper deposition rate is limited by the onset of “spitting” from the effusion cell. Spitting can result from convection flows (sometimes inaccurately referred to as “boiling”) established in a heated melt, which increase in ferocity as temperature is raised, resulting in liberation of droplets of liquid that can reach the substrate and result in defects ranging from metal droplets to dried chemically reacted droplet alloys that in some applications are often described as “oval defects” for their identifying geometry. Source spitting can also occur at lower temperatures when some evaporate condenses on the crucible wall and collects. This collection is aggravated when using heat transparent materials such as PBN because the walls of the crucible above the melt are not as hot as the melt itself, encouraging condensation of droplets on the walls. These condensed droplets can fall back into the melt “splattering” liquid out of the cell. Either of these mechanisms, or other similar mechanisms known in the art, can result in source spitting that can incorporate defects in the deposited film. At a moderate rate of deposition, spitting defect production has been reduced by heating the orifice, or lip as in “hot lip” designs of the crucible to prevent material condensation or using reentrant crucible shapes. These designs suffer from inefficient heat absorption, and limited temperature differential compared to the melt region providing only modest reduction of spitting compared to standard open cell designs.
A “campaign length” is the time an MBE or PVD system can be used for growth before the system has to be opened for maintenance, most often material reloading. Almost all currently used effusion cells utilize a single opening design, wherein the material is evaporated through the same opening through which the material is loaded. This requires that the effusion cell be removed from the system to reload expended material. In processes requiring high purity, such as deposition of semiconductor material where one impurity in approximately 100 trillion semiconductor atoms is considered “impure,” this breaking of vacuum and subsequent system contamination via H2O, CO, O2, CO2 and other contaminating gases found in the atmosphere, causes an expensive and time consuming “post-maintenance cleanup” cycle to be employed. Often, the entire system is baked at elevated temperatures as high as 250° C. for several days to drive out the impurities into various pumps. This is deemed as one of the primary industrial limitations of the MBE process and is the primary reason many manufacturers select competitive technologies instead of MBE. In the select few sources that do not load materials through the same opening through which they evaporate the material, the design is such that breaking vacuum to load the materials is still required, resulting in similarly limited campaign length.
Uniformity of the deposited film on the target substrate is the statistical and geometric variation of the thickness of the film or atomic composition of an alloy, if so composed, as a function of position on the substrate. PVD systems, such as MBE systems, are known for producing highly uniform films with standard deviations of thickness and composition being 1% or less in some cases. Given that the flux distribution versus angle to the effusion cell axis typically follows a cosine to a power (that power typically being less than ˜3), high uniformity in an MBE system is obtained by increasing the source-to-substrate distance to a large enough distance so as to obtain a nearly flat flux distribution as shown in FIGS. 1A and 1B. However, since the flux arriving at the substrate reduces by a factor of 1/LSS2, where LSS is the substrate-to-source spacing, this common practice also reduces the deposition rates substantially. Positive draft crucibles have been used to improve uniformity, but these have reduced capacity compared to straight wall crucibles and exhibit well-known long-term flux instabilities known as “depletion effects,” wherein the surface area of the materials being evaporated change as the material is depleted, changing the flux over time, even at a constant temperature. All open-ended crucibles, whether positive draft (conical) or straight walled, exhibit some measure of long-term flux instabilities due to depletion effects and are prone to short-term flux instabilities when the external shutter is opened or closed. These short-term external shutter flux transients are well known to result from the transient cooling or heating of the effusion cell when the partially heat-reflecting shutter is removed from the effusion cell opening, thus, allowing more (open shutter) or less (closed shutter) heat loss from the cell that is compensated for by transient variations driven by the Proportional Integral Derivative (PID) control system, which applies more (open shutter) or less (closed shutter) power to the resistive filaments to return to the desired constant temperature.
It is known that the flow leaving an effusion cell containing a vapor or gas with pressure P and proceeding into a vacuum via a restricted, sufficiently thin opening (an orifice) of cross-sectional area A is given by:J=PA/(2πmkT)1/2,  Equation 10)where the opening is sufficiently thin to be classified as an orifice by having a thickness much thinner than the opening width and with: J=flow, m=molecular mass in kG of the evaporated species, k=Boltzmann constant, T=absolute temperature in the cell, P=pressure in the cell linked with the temperature T related by a law in the form:P=Ye(−Ea/kT),  Equation 11)Y being a characteristic constant of the evaporated material and Ea is an activation energy associated with the evaporation or sublimation process.
The exact flux of molecules at a distance from the orifice, LSS, into the vacuum is a complex function of many variables, but is known in the art. Some of these variables include J, orifice size, A, orifice shape, and even local pressure (P is a function of position z, where 0≤z≤LSS), which changes as the molecular beam expands or focuses as it processes into the vacuum to substrate location point LSS.
Lambert's approximation is thus useful and says an incident flux F on a substrate located at a distance LSS from the orifice diminishes as the square of the distance, LSS, from the orifice:F=HJ/(LSS2),  Equation 12)where H is a proportionality factor.
Known methods of creating this flux typically involve a crucible containing the material to be vaporized, a filament heater to apply heat to the crucible, and a beam-blocking mechanism (shutter or valve). Crucibles can be made from a variety of materials but are selected for their ability to not react chemically with the source material to be vaporized, and minimal outgassing of undesirable contaminant gases. The crucible is most often a container with one open end, but in some cases, is a sealed container with a nozzle or orifice designed to constrict or shape the flux leaving the cell.
The vast majority of effusion cells use a mechanical shutter mounted exterior to the crucible consisting of a blade placed in the path of the molecular beam of the flux leaving the effusion cell. These exterior mounted shutters reduce the delivered flux, but at higher pressures where scattering or chamber desorption occurs, this flux reduction may only be a factor of ˜10 when in the closed position. This blade can be made of W, Ta, Mo, PBN, graphite or any other material of sufficiently limited reactivity with the evaporate material and is typically not heated, resulting in a continually increasing thickness of deposited material on the shutter. The deposited material can accumulate to such thicknesses as to cause shutter actuation problems (sluggish or locked behavior) and can even touch the cold surfaces surrounding the shutter, forming a “solidified material weld” between the shutter and the cold surfaces, preventing shutter actuation. The blade is typically actuated via a linear retraction/insertion mechanism or a rotary motion along an arc path substantially less than 360°, typically from 90° to 180°. Closing an open shutter requires the shutter to reversely traverse the path it traveled during its opening cycle, which is a slow and often unwieldy process.
Likewise, in a rare number of effusion cells, a plunger style valve is used to plug and then open the effusion cell crucible's cylindrical tube, so as to cut off and then restore the molecular flux. Such a design allows for a heated valve body, which minimizes the material accumulation and provides a more positive closing, which lowers the amount of flux “leaking” past the valve/shutter. However, even these plunger valve methods of flux modulation require the same path to be reversely traversed, resulting in slow flux modulations similar to the external shutter configurations. These methods of molecular flux interruption were designed to simply start and stop the beam occasionally and are known in the MBE field to produce difficulty when rapid, frequent shutter/valve actuations are required. Methods that require rapid flux modulations include the growth of multiple layers of thin alternating compositions known as superlattices or when supplying frequent pulses of metal to increase surface migration as found in metal-modulated epitaxy (MME) processes, such as those disclosed in, for example, U.S. Pat. No. 9,142,413 titled “Systems and Methods for Growing a Non-Phase Separated Group-III Nitride Semiconductor Alloy.” Thus, neither the exterior-mounted “blade style” shutter nor the internally mounted “plunger style valve” can provide the needed speed of actuation desirable when rapid pulse-modulated flux growth is required. These temporal constraints of the shutter/valve systems are exacerbated as the deposition rate increases. For example, in the MME case, shutter actuation may be every 2 seconds for a growth rate of ˜1-2 μm/hour, but increase in frequency to every 0.1 to 0.2 second for a growth rate of ˜10-50 μm/hour.
One of the primary reasons for using an MBE system is to maintain an ultra-high vacuum background to minimize the incorporation of undesirable impurities. Most often, these impurities are in the form of undesirable oxygen and carbon gases, primarily from heated metals that act as near infinite sources of these undesirable contaminant gases. Extreme care is given to the maintenance and operation procedures to ensure very low base pressures are achieved daily. For example, a commercial MBE system used to grow III-Nitride materials may commence each operational day with a base pressure of ˜6-8×10−11 Torr (near the lower limit of what can be read by an ion gauge pressure sensor). However, as soon as any of the effusion cells or substrate heaters are increased to their operational temperature from idle values (about 200° C.), the base pressure in the system may rise from this quiescent value to as high as 10−9 Torr (and, momentarily, even higher).
MBE is performed using effusion cells heated by resistive metal filaments and substrate holders typically made from metallic Ta, W, or zirconia-stabilized Pt. These resistive metal filaments, ceramic parts holding the resistive metal filaments, and gas trapping rolls of metal used to reflect heat and minimize the amount of heat escaping the effusion cell or substrate heater, however, can result in introduction of impurities into the evaporate flux. Metals are an infinite source of carbon- and oxygen-bearing gases and tend to outgas CO, CO2 and O2 gases, and other undesirable elements, which need to be pumped away or they will be incorporated into the growing film. Furthermore, the substrate heaters and effusion cells are in direct line-of-sight to the growth substrates. Thus, all the elaborate cryoshields and gettering pumps, useful for scattered or desorbed gases, have little effect on lowering these gas concentrations before they encounter the growth substrates.